fix(epd7in5_v2): match Waveshare reference framebuffer polarity

src/epd7in5_v2/mod.rs

@@ -131,7 +131,16 @@ where
         delay: &mut DELAY,
     ) -> Result<(), SPI::Error> {
         self.wait_until_idle(spi, delay)?;
-        self.cmd_with_data(spi, Command::DataStartTransmission2, buffer)?;
+        // Waveshare's reference C demo (EPD_7in5_V2.c::EPD_7IN5_V2_Display)
+        // sends the framebuffer to DTM1 (0x10) raw and to DTM2 (0x13) bitwise-
+        // inverted. The user-facing convention is bit 1 = white; the panel's
+        // DTM2 register expects bit 0 = white (datasheet §22, KW mode with
+        // NEW/OLD, DDX=00). Writing both DTM1 and DTM2 with opposite polarity
+        // forces a full LUTKW/LUTWK transition for every pixel, producing
+        // strong contrast. Without this, every framebuffer renders inverted.
+        self.cmd_with_data(spi, Command::DataStartTransmission1, buffer)?;
+        self.command(spi, Command::DataStartTransmission2)?;
+        self.interface.data_inverted(spi, buffer)?;
         Ok(())
     }
 
@@ -169,8 +178,11 @@ where
         self.wait_until_idle(spi, delay)?;
         self.send_resolution(spi)?;
 
+        // Match Waveshare's `EPD_7IN5_V2_Clear` (DTM1=0xFF, DTM2=0x00) so
+        // every pixel transitions black->white via LUTKW. See `update_frame`
+        // for the polarity rationale.
         self.command(spi, Command::DataStartTransmission1)?;
-        self.interface.data_x_times(spi, 0x00, WIDTH / 8 * HEIGHT)?;
+        self.interface.data_x_times(spi, 0xFF, WIDTH / 8 * HEIGHT)?;
 
         self.command(spi, Command::DataStartTransmission2)?;
         self.interface.data_x_times(spi, 0x00, WIDTH / 8 * HEIGHT)?;

src/interface.rs

@@ -89,6 +89,23 @@ where
         self.data(spi, data)
     }
 
+    /// Sends data with bytewise bitwise-NOT applied. Streams via a stack chunk
+    /// to avoid heap allocation. Required for displays whose DTM2 register
+    /// expects bit polarity inverted from the user-facing framebuffer (e.g.
+    /// 7.5" V2, where Waveshare's reference C demo
+    /// `EPD_7in5_V2.c::EPD_7IN5_V2_Display` applies the same `~` before 0x13).
+    pub(crate) fn data_inverted(&mut self, spi: &mut SPI, data: &[u8]) -> Result<(), SPI::Error> {
+        let _ = self.dc.set_high();
+        let mut chunk = [0u8; 256];
+        for source in data.chunks(chunk.len()) {
+            for (index, &byte) in source.iter().enumerate() {
+                chunk[index] = !byte;
+            }
+            self.write(spi, &chunk[..source.len()])?;
+        }
+        Ok(())
+    }
+
     /// Basic function for sending the same byte of data (one u8) multiple times over spi
     ///
     /// Enables direct interaction with the device with the help of [command()](ConnectionInterface::command())